ios - 无法解析“NIL”值（DispatchQueue、VNCoreMLRequest）

Question

我正在尝试更新一个类变量（预测），然后将该变量用作segue（AnalyzeSegue）的数据。但是，即使我在函数 (detectScene) 中更新了类变量，类变量也是nil为 segue 做准备期间的一个值，导致我的应用程序崩溃。

大部分代码是通过本教程（只是为我的模型定制）：

https://www.raywenderlich.com/577-core-ml-and-vision-machine-learning-in-ios-11-tutorial

class PreviewViewController: UIViewController {

    var prediction: String?

    @IBOutlet weak var Photo: UIImageView!


//This button is what starts the segue into the next screen(
@IBAction func analyze(_ sender: Any) {}

override func prepare(for segue: UIStoryboardSegue, sender: Any?){

        if segue.identifier == "saveSegue"{
                           "save photo"}

        else if segue.identifier == "AnalyzeSegue"  #Analyze button

        {

            let AnalyzeVC = segue.destination as! AnalyzeViewController
              let ciImage = CIImage(image:Photo.image!)
             self.detectScene(image: ciImage!)
            print(self.prediction!) #nil value
            if let hasPrediction = self.prediction {
                AnalyzeVC.predictionLabel.text = hasPrediction
                AnalyzeVC.imagePreview.image = Photo.image}


}

检测功能：

extension PreviewViewController{
    func detectScene(image: CIImage) {
        var localPrediction: String?

// Load the ML model through its generated class
        guard let model = try? VNCoreMLModel(for: test2().model)
            else {
            fatalError("can't load Places ML model")
                    }
        let request = VNCoreMLRequest(model: model) { [weak self] request, error in
            guard let results = request.results as? [VNCoreMLFeatureValueObservation]

                 else {
                    fatalError("unexpected result type from VNCoreMLRequest")
                        }
            let topResult = results.first?.featureValue.multiArrayValue
             DispatchQueue.main.async { () in
                var max_value : Float32 = 0        
                for i in 0..<topResult!.count{
                    if max_value < topResult![i].floatValue{
                        max_value = topResult![i].floatValue
                        localPrediction = String(i)

                                                            }

                                                } //#end of loop

                self?.prediction = localPrediction #update self?.prediction
                print(self?.prediction, "Inside Function") #seems to have value
                                        }   //#end of dispatch Queue 
       } //#end of closure

        let handler = VNImageRequestHandler(ciImage: image)
          DispatchQueue.global(qos: .userInteractive).async {
                do {
                    try handler.perform([request])

                   } catch {print(error)}


                                                        }

                                                         } //end of function

                                                                } //end of extension

因此，该detectScene函数应该更新类变量prediction，并且该信息用于准备 seugue 函数。该函数似乎是更新值，因为我在函数中有一个打印语句正在打印出一个可选值：

  print(self?.prediction, "Inside Function")

谢谢您的帮助。

标签： iosswiftoptionaldispatch-queue